exec-env/exec-file: support "--" to separate command to run

An issue I am facing regularly is that I need to create an actual command that is being executed with exec-env or exec-file with variables. Due to the need to put the whole command line into a single argument it becomes very and error-prone due to quoting issues.

A simple example:

my_url="${api_baseuri}/${resource_path}"
sops exec-env test.env 'curl -s --user "${API_USER}:${API_PASSWORD}" "'"${my_url}"'"'

If sops could introduce special support for the -- (double-dash) argument, we could say that everything after it becomes a part of the command to execute and it could look like this instead:

sops exec-env test.env -- curl -s --user '${API_USER}:${API_PASSWORD}' "${my_url}"

For manual/interactive command building, this would also allow to auto-complete file names in the shell we're working in.

Of course, the user would still need to ensure that variable expansion doesn't happen in the shell (using single quotes instead of double-quotes), but that has less error potential than quoting (at least for me).

Start by tracing the exec-env and exec-file command entry points and how they currently receive the command argument. Implement the documented behavior that arguments after -- form the command to execute, then verify it with the bash examples in the issue and the existing command tests.